The present invention relates to an arrangement in an oil-free rotary gas compressor which has a high, built-in pressure ratio and which is provided with means for injecting liquid thereinto, preferably water, for the purpose of cooling the gas under compression.
By high, built-in pressure ratio is meant here and in the following a ratio which is greater than about 4:1.
Oil-free gas compressors are commonly used to compress air from atmospheric pressure to pressures in the region of from 8 to 12 bars. In known compressors of this kind, considerable quantities of water are injected, in order to restrict the terminal temperature of a compression stage to about 50.degree. C., at an incoming air temperature of about 20.degree. C. The rise in temperature corresponds to a mass ratio, water/air, of 10:1 or thereabove, although it is known to limit this ratio to 1.4:1. The amount of water injected into the compressor per unit of time would, if it were to be consumed, constitute a substantial part of the operating costs. Consequently, the water is removed and re-cycled subsequent to being cooled, and optionally also reconditioned. The water-removal system, which also incorporates a quantity of buffer water and the conditioning system, which protects against, inter alia, the formation of bacteria, lime deposits and acidification, is highly space consuming and should be constructed from a corrosion resistant material. The system, when connected to a water injection compressor, is therefore expensive. Water injection also necessitates a marked reduction in compressor speed, with a subsequent reduction in capacity.
In the case of a corresponding dry single-stage compressor, outlet temperatures in the order of 350.degree.-400.degree. C. are reached, resulting in large temperature gradients in the various compressor components, and therewith excessive play therebetween and poor efficiency. In order to overcome this, it is necessary to compress the gas in two or more stages and to cool the gas between consecutive stages. This solution, however, results in a compressor of large dimensions, particularly when the cooling arrangements are included in the dimensions of the apparatus as a whole.
The advantages and disadvantages encountered with liquid injection compressors and dry compressors have been detailed in "Mechanical Engineers' Handbook" (1951), McGraw-Hill Book Company, Inc. On page 1879 of this publication there is also proposed a solution which is intended as a compromise between the small dimensions and high speeds of the dry compressor on the one hand and the beneficial cooling effect of the liquid injection compressor on the other. This compromise solution comprises injecting a restricted quantity of water into the inlet of a high speed single-stage compressor, so that all the water is vaporized by the heat generated during the compression process. It has been found that this will enable the outlet temperature to be restricted to 100.degree.-150.degree. C., while reducing temperature gradients and play and improving efficiency to a corresponding degree in comparison with a dry single-stage compressor. The efficiency is lower, however, than that of the initially mentioned water injection low speed compressor.
Thus, according to the aforesaid handbook, an ineffective area is found with regard to the quantity of water injected per unit of time, namely between the limited liquid injection and the injection of considerable quantities per unit of time. This conclusion has prevailed for approximately 25 years.
The object of the present invention is to provide an improvement in oil-free rotary-gas compressors with liquid injection in relation to the total capacity requirement of the compressor.
Contrary to the practice documented in the aforesaid handbook, this object has been achieved in accordance with the present invention by constructing the liquid injection arrangement in a manner which will enable the liquid to be injected in a weight quantity relative to the weight quantity of the gas supplied which is greater, although not more than four times greater, than that required to achieve complete vaporization of the liquid during the compression process. The result of this improvement is that in the final stage of the compression process, water which has not vaporized will lie on the surfaces of the compression chamber, these surfaces being colder than the surroundings, and there seal leakage through the play between the actual rotors themselves and between the compressor housing and the rotors, while keeping the amount of water in the compressor outlet is so small as to produce but small dynamic losses, this water being removed with the aid of a simple condensation separator and either discharged to sewage or recycled through a simple recycling system. If such a system is required, it need only be of simple and inexpensive construction, due to the small amount of water concerned and also due to the fact that there is less need to clean the system than in the case of conventional water injection systems service requirements are, naturally, considerably less.
Further characteristics of arrangements according to the invention are set forth in the claims.